A tandem type color laser printer in which four photosensitive drums for yellow, magenta, cyan and black are laterally arranged is known as a color laser printer of an electrophotographic system. In this tandem type color laser printer, electrostatic latent images are written on the photosensitive drums, which are then developed with toners of the various colors. As a result, toner images of various colors are formed substantially at once on the photosensitive drums, and these toner images are transferred in a color-superposed state on a sheet material passing the photosensitive drums in order, so that color images can be formed at substantially the same speed as a monochromatic image obtained by a monochromatic laser printer.
In such a tandem type color laser printer, it is necessary that latent images corresponding to various colors with respect to the four photosensitive drums be formed. However, when four scanners are provided correspondingly to the four photosensitive drums, it becomes unavoidable that the cost of the apparatus and the dimensions thereof increase. Therefore, a structure for forming electrostatic latent images by putting four laser beams corresponding to the various colors into the same reflective surface of a polygon mirror, carrying out the polarization scanning in the main scanning direction while separating the optical path toward the auxiliary scanning direction, and thereby forming electrostatic latent images corresponding to the various colors on the four photosensitive drums by one optical scanner. (Refer, e.g. JP-A-2003-295079.)
However, in the structure in which four laser beams are put in the same reflecting surface of the polygon mirror, the incident positions of the various laser beams on the same reflecting surface become close to one another, and a distance between the optical paths of the various beams reflected on the same reflecting surface become narrow unless the dimensions of the apparatus are greatly enlarged. Therefore, the optical members, such as a reflecting mirror provided in the portion of the optical path of the reflected laser beam have to be placed in predetermined positions with a high accuracy so as to avoid the interference (for example, reflection and cutoff) of the laser beam with the optical paths through which the laser beams pass. As a result, extremely much labor and much time are needed for the arrangement of the members in predetermined positions with a high accuracy.
Therefore, an object of the present invention is to provide an optical scanner capable of reducing the labor and time needed to arrange the optical members without enlarging the dimensions of the apparatus, and an image forming apparatus provided with the same.